The '-trace' and '--trace' spellings are only both supported in qemu
binary, while for qemu-nbd or qemu-img only '--trace' spelling is
supported. So for the consistency of trace option invocation, we
should use double-dash spelling in our documentation.

This's also mentioned in
https://wiki.qemu.org/BiteSizedTasks#Consistent_option_usage_in_documentation
.
Signed-off-by: NYaowei Bai <baiyaowei@cmss.chinamobile.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 1530674247-31200-1-git-send-email-baiyaowei@cmss.chinamobile.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>